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1 Department of Kinesiology, University of Waterloo, Ontario N2L 3G1, Canada; 2 Division of Cardiology, Department of Medicine, University of Oulu; and 3 Merikoski Rehabilitation and Research Center, Oulu 90100, Finland
tk;1Passive head-up
tilt and exercise result in specific changes in the spectral
characteristics of heart rate (HR) variability as a result of reduced
vagal and enhanced sympathetic outflow. Recently analytic methods based
on nonlinear system theory have been developed to characterize the
nonlinear features in HR dynamics. This study was designed to assess
the changes in the fractal and complexity measures of HR behavior
during the passive head-up tilt and during exercise. Fractal exponent
(
1) and approximate entropy (ApEn), measures of
short-term correlation properties and overall complexity of HR,
respectively, along with spectral components of HR variability were
analyzed during a passive head-up tilt test (n = 10)
and a low-intensity steady-state exercise (n = 20) in
healthy subjects. We observed that
1 increased during the tilt test (from 0.85 ± 0.22 to 1.48 ± 0.20;
P < 0.001) and during the exercise (from 1.00 ± 0.22 to 1.37 ± 0. 14; P < 0.001). ApEn also
increased during the exercise (from 1.04 ± 0.11 to 1. 11 ± 0.08; P < 0.05), but it did not change during the tilt
test. The normalized high-frequency spectral component decreased and the low-frequency component increased similarly during both the exercise and the tilt test (P < 0.001 for all).
Exercise and passive tilt result in an increase of short-term fractal
correlation properties of HR dynamics, which is related to changes in
the balance between the low- and high-frequency oscillations in
controlled situations. Overall complexity of HR dynamics increases
during exercise but not during passive tilt.
variability; approximate entropy; detrended fluctuation analysis
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